More specifically, the invention relates to the operating conditions of the regeneration gas heater associated with such a unit or the operating conditions of a heater that exhibits the same characteristics in terms of the temperature levels and fluids involved.
Mixtures of the syngas type may be obtained in a number of ways, and in particular:                by CO2 or steam reforming,        by partial oxidation        by hybrid methods such as the ATR (Auto Thermal Reforming) method which is a combination of steam reforming with partial oxidation, from gases such as methane or ethane,        by gasification of coal,        or recovered as residual gases downstream of acetylene manufacturing units.        
Aside from the hydrogen and carbon monoxide by way of main components, numerous impurities such as carbon dioxide, water or methanol often form part of the syngases.
Among purification methods, the method of the TSA (Temperature Swing Adsorption) type is a cyclic method in which each of the adsorbers alternate adsorption steps during which the impurities are held in the adsorbent and regeneration steps during which use is made in particular of a heating phase in order to extract the impurities from the adsorbent. This heating is generally performed by means of a gas known as a regeneration gas which originates from a cryogenic treatment, that is to say, in this case, a hydrogen-rich fraction, a residual gas, a mixture of the two, or a fraction of the purified syngas. In all of these cases, the regeneration gas contains at once hydrogen, carbon monoxide and methane, but in varying proportions. It is also generally used thereafter to cool the adsorbent mass down to its adsorption temperature.
The typical operating cycle for this kind of unit is described in document WO-A-03/049839.
The units for purification methods of the TSA type are generally engineered to obtain a syngas of cryogenic quality, that is to say that, when said syngas is cooled in the cold box, any deposits of impurities are small enough that satisfactory operation of said cold box can be guaranteed for several years, therefore without plugging, and without thermal deterioration to the heat exchange line and without risk to equipment safety.
What that means is a residual CO2 content generally of the order of 0.1 mol ppm maximum and contents which are even lower than a mol ppb level for the other impurities.
In order to limit intervention on these purification units, they are also engineered to have enough of an initial margin that they can operate correctly for several years without the need to replace the adsorbents.
Despite all these precautions, it has been found that the life of these units is appreciably shorter than initially forecast.
In normal operation, a CO2 analyzer checks the purity of the gas produced. It allows the cycle to be modified, for example the adsorption phase to be shortened, if premature CO2 break-throughs associated with a degradation in purification unit performance as mentioned previously are detected. Nonetheless, despite these precautions, it is found that the separation performance of the cold box that performs cryogenic separation of the syngas deteriorates after a few years of operation.
This lack of performance is attributable to a degradation of heat exchange as a result of solids being deposited on the heat exchanger plates.
Shutting down the unit and heating it (to deice it) does solve the problem but this is, of course, expensive if this is not a pre-programmed shut down, because it forces the unit, and therefore production, to be shut down.
Given the margins of safety on the exchangers which are adopted during the design of the cold boxes, these effects are not felt until after the units have been running for a relatively long period of time, longer than 1 year, and more generally of the order of 2 to 3 years. This means that it is impossible to tell whether the entrainment of traces of impurities by the purified syngas, in theory water and CO2, into the cold box occurs after more than one year of operation, after several months, or after operating for just a few weeks.
It has been reported that this degradation is the result of chemical reactions between the adsorbent and the adsorbate and/or of reactions between the components of the syngas which reactions are encouraged by the adsorbent.
The reactivity of H2/CO mixtures at high temperature is indeed well known. Thus, document U.S. Pat. No. 5,897,686 teaches that a number of reactions occur during the repressurization phase of the purification, which is a substep of regeneration. Mention is made therein of the following two reactions in particular:methanation: CO+3H2→CH4+H2Othe Boudouard reaction: 2CO→C+CO2 
According to that document, the problem encountered is linked with the formation of water in the adsorbent and the recommended solution is to add a bed of 3A sieve to the top of the adsorber which sieve because it does not absorb CO prevents the in-situ formation of water. That document recommends a regeneration temperature of between 100° C. and 400° C., which conventionally corresponds to a heater skin temperature of the order of 150/200° C. to 450/500° C.
Certain chemical reactions can also be catalyzed by deposits of secondary constituents at the surface of the adsorbents. Deposits of metals such as iron, nickel, copper, etc. facilitate the abovementioned reactions. In the case of some of them, their origin is due to the breakdown of metal-carbonyls formed earlier in the purification process.
Gradual poisoning of adsorbents by traces of impurities that cannot be or can be only imperfectly regenerated is also a plausible hypothesis, given the very high number of products of secondary reactions that may occur in synthesis reactors, that may originate from the coal or natural gas used as a raw material, or that may be entrained by earlier pre-purification methods, such as the methanol scrubbing or amine scrubbing.
Document WO-A-2006/034765 describes a method of purifying a stream of a gas rich in carbon monoxide and in hydrogen, in which method the stream of gas is brought into contact with an adsorption layer containing silica gel and the adsorption layer is regenerated using a gas the temperature of which ranges between 70° C. and 150° C., which corresponds to a heater skin temperature of the order of 150° C. to 200/250° C.
The skin temperature of the heater is defined as the temperature to which the regeneration gas is subjected as it passes through the heater. It is normally kept constant throughout the pressure cycle in order to limit thermal shocks on the exchanger. In the most general case which involves using steam as a means of heating, the steam supplied to the exchanger is kept open. When the regeneration gas is no longer flowing through the exchanger, the temperature gradient across the heat exchange surface becomes practically zero and the skin temperature in practice becomes very close to the steam condensing temperature. It may be considered to be equal to this same temperature. In normal operation, that is to say during heating periods, the temperature gradient across the heat exchange surface is not zero but remains of a secondary order by comparison with the total gradient between the temperature of the steam and the temperature within the gas that is to be heated. The skin temperature can then also be likened to the steam condensing temperature. Use is generally made of saturated or slightly superheated steam. Even when the steam is superheated to a greater extent, for example when the temperature at which it is available is 50° C. higher than its condensing temperature, most of the heat exchange occurs at a skin temperature close to said condensing temperature.
It is also known that, for a given thermal power (Q), the area (S) of heat exchange surface to be installed is in inverse proportion to the temperature differential ΔT between the skin temperature of the heating surface and the temperature of the regeneration gas in contact with the exchange surface.
Hence, it will be readily understood that, in order to reduce the area of heat exchange surface needed, and therefore the investment, it is necessary to use a skin temperature T1 that is as high as possible. For example, in a refinery, a chemical or petrochemical works, in order to heat a fluid to a temperature of 170° C., it is common practice to use steam at 250/300° C. or even higher.
Despite all the poisoning hypotheses, the main reason why impurities are introduced into the cold box has not yet been clearly identified.
Nonetheless, one of the problems that arise is that of supplying a syngas of cryogenic quality without having to intervene prematurely on the purification units and/or on the cold box by proposing an effective method for purifying an H2/CO mixture containing at least one impurity, in such a way as to avoid or minimize parasitic reactions.